1. Field of the Invention
The present invention relates to a method and apparatus for image processing, and more particularly to a method and apparatus that effectively corrects a line noise produced due to a relatively high-speed clock signal.
2. Discussion of the Background
Image processing is an important part of image forming apparatuses such as copying machines, facsimile machine, etc., but suffers from a high frequency radiation noise, particularly in a place where image data is input with a line image sensor such as a CCD (charge-coupled device). This problem is due to an application of a high-speed clock signal used to control image processing.
Frequency dispersion is one attempt to avoid the above-mentioned problem. Frequency dispersion is a technique for continuously modulating an oscillating frequency of a clock signal in a predetermined cycle so as to prevent high frequency noise from occurring at a specific frequency.
However, using the above-mentioned technique, another problem that doesn't appear in a digital signal stream occurs in an analog signal stream. In particular, when a CCD line sensor is used, a frequency dispersion cycle is not easily synchronized with an operation cycle of the CCD line sensor. Further, the CCD line sensor is driven by a clock signal that is frequency-dispersed. Therefore, an output signal waveform from the CCD line sensor is inevitably effected by such frequency-dispersed clock signal. Also, sampling positions of the CCD line sensor may be effected. Consequently, the CCD line sensor responses to the frequency dispersion cycle in a way that an output image signal includes noise asynchronous to a reference signal of a line scanning operation. As a result, this noise causes critical black lines in an image.
Two attempts have been conducted to solve the above-mentioned noise problem. A first method is described in Japanese unexamined patent publication No. 2000-138805, in which the cycle of the frequency dispersion is reset by a main scanning synchronous signal of the CCD line sensor in each scanning operation. This technique attempts to synchronize the noise with the reference clock signal of the scanning operation to avoid variable noises and to stabilize an appearance of the noise in an image, thereby improving an appearance of the image. In this method, however, the dispersion cycle is provided with an asynchronous reset signal and therefore a reduction of the radiation noise level cannot be stably maintained.
A second method is described in Japanese unexamined patent publication No. 2000-22959, in which noise elements in an image signal for one line are previously obtained and the image data is corrected for noises with reference to the previously obtained noise elements. This method, however, requires a considerable increase in a memory capacity.